Chaos exhibited, for example, in a sequence of numerical values, can be described in a variety of ways. Simply stated, chaos in a sequence of numbers is indicative of the sequence of numbers that is at least initially deterministic, but that is complex and that displays apparent random characteristics and becomes unpredictable in a long term. Generally, a chaotic system and/or a chaotic model are nonlinear and very sensitive to initial input conditions. Chaos is more fully described below.
It has been recognized that electrical signals produced by a human heart reflect activity of a nonlinear dynamical system, which may be described using chaos theory. Chaos in general and chaos of electrical signals produced by the heart have been described, for example, in U.S. Pat. No. 5,792,062, issued Aug. 11, 1998, and in U.S. Pat. No. 5,938,594, issued Aug. 17, 1999, both of which are incorporated by reference herein in their entireties, and both of which are assigned to the assignee of the present invention.
Accordingly, the human heart may be referred to as nonlinear dynamical or chaotic system. Dynamical systems, such as the heart, can exhibit both periodic and chaotic behavior depending upon certain system parameters. These parameters appear as constants in mathematical equations describing the heart system. However, the chaotic behavior exhibited by the heart is not immediately obvious when looking, for example, at an electrocardiograph (ECG) signal.
One way to observe the chaotic behavior of the heart is to plot interbeat spacing (i.e., heart interbeat interval) or its reciprocal (i.e., heart rate) at a time n against the interbeat spacing (or heart rate) at time n+1. Such a plot is referred to as a Poincare map or a return map. However, one problem with this technique is that a relatively large amount of data is required to provide an accurate representation of the heart system.
In general, problems arise in collecting large amounts of data from biosystems. For example, collection of a relatively large amount of bioelectric data on a human heart requires a human to wear a monitor or sensor for a relatively long period of time. Moreover, relatively large processing power is required to analyze the large amount of data retrieved from the human subject or other biosystem. The need to process such large amounts of data makes it relatively difficult to provide a real time processing system. Furthermore, a relatively large amount of storage capacity is required to store the large amount of collected data. Most importantly, analyses that require large amounts of data collected over a protracted period are susceptible to time-dependent changes in the system states that may occur during the data collection period, hence such analyses cannot detect any rapid changes in systems states.
Chaotic behavior of the heart can be linked to sympathetic or to parasympathetic behavior of the autonomic nervous system.
It is known that many diseases such as sleep apnea and heart failure present with increased sympathetic behavior. However, detection of sympathetic behavior has been difficult and invasive techniques have been used.
Attempts have been made to detect physiological abnormalities by measuring variations of a quantifiable characteristic of a patient's heart. For example, attempts have been made to detect sleep apnea by measuring a high frequency portion and/or a low frequency portion of a power spectrum of a heart rate and/or of a heart interbeat interval of a patient.
However, the high frequency portion of the power spectrum is linked to parasympathetic behavior of the autonomic nervous system and is not a direct indicator of sympathetic behavior in itself. Evidence of the relationship between the high frequency portion of the power spectrum and parasympathetic behavior can be seen in clinical and experimental observations of autonomic maneuvers such as vagus nerve stimulation, muscarinic receptor blockage, and vagotomy, for which the efferent vagal activity is a major contributor to the high frequency portion of the heart rate or interbeat interval power spectrum.
The low frequency portion of the power spectrum appears to reflect both sympathetic and parasympathetic activity and therefore, is also not a good indicator of sympathetic activity.
It would be desirable to provide a method and apparatus to detect physiological abnormalities by measuring variations of a quantifiable characteristic of a heart of a patient, where the quantifiable characteristic is linked to sympathetic behavior of the autonomic nervous system. It would be desirable to provide such a method and apparatus that have high sensitivity and high specificity in detecting the physiological abnormality. In other words, it would be desirable to provide such a method and apparatus that can detect even mild amounts of the physiological abnormality and with a low rate of false detections. It would be desirable to provide such a method and apparatus that are non-invasive. It would be desirable to provide such a method and apparatus that are robust, namely, that have an ability to reject artifacts, like disturbances and noise.